Christou, Aristos; (2024) Predicting Complex Wave-structure Interaction with the Method of Large-eddy Simulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A three-dimensional numerical wave tank based on the in-house open-sourced large eddy simulation code Hydro3D is introduced. The code employs the level set and immersed boundary methods to capture the evolution of the water surface and to account for solid structures in the numerical tank, respectively. The spatially-filtered Navier-Stokes equations are solved on a staggered Cartesian grid using the finite difference method while time advancement is achieved using the fractional-step method based on a three-step Runge-Kutta scheme. Velocities and pressure are coupled with the Poisson equation and its solution is obtained via a multi-grid technique. The proposed NWT generates solitary, linear or non-linear Stokes waves up to 5th order as well as cnoidal and focused waves using Dirichlet boundary conditions utilising the velocities and wave elevations from analytical solutions of wave theories or by introducing experimental measurements. On the opposite end, waves are absorbed using the artificial damping or the relaxation method to avoid reflected waves interacting with incident wave trains, similar to a physical flume. To evaluate the accuracy of the code, the NWT is employed to simulate benchmark cases to examine the performance of each sub-module, for example, the level set method based on the progression of a run-up wave, a flow past a sphere at low Reynolds numbers for the immersed boundary method and the surface tension on stationary or rising spherical bubbles. The NWT is then applied to predict the progression and damping of monochromatic waves to examine the capability of the numerical tank to generate accurate wave conditions. Additionally, the current code is employed to simulate previous laboratory experiments of a solitary wave propagating over an infinitely wide flat plate or waves over submerged steps and trapezoidal bars. Then three-dimensional simulations are conducted to examine the effect of three-dimensionality and the performance of large eddy simulations on realistic wave structure interactions. Among others, simulations of a submerged finite square plate of various angles of attack are conducted and results are compared with the corresponding two-dimensional case. Additionally, experiments of focused waves interacting with a piercing cylinder and depth-varying currents of various directions are reproduced by employing the current numerical wave tank and simulations are compared with experimental measurements. Comparisons of numerically predicted and measured water-level elevations, local velocity and pressure fields and forces acting on structures under the influence of incoming waves agree well in all simulations conducted and confirm that the LES-based NWT can accurately predict three-dimensional wave-structure interaction while other available CFD methodologies are insufficient or limited due to the model's primary assumptions.

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